1. Field of the Invention
The invention relates to parachutes and is directed more particularly to a reefing assembly for use in conjunction with a circular parachute to reduce parachute opening shock load.
2. Description of the Prior Art
Parachutes are often used to deliver cargo to remote and inaccessible rough terrain areas. They are also used to resupply military units on land and at sea. Deployment of parachutes usually occurs after a cargo aircraft reduces its speed from its cruise velocity to some lower velocity at low altitude to allow deployed cargo parachutes to survive shock loads generated during the parachute opening phase of the airdrop. High altitude deployment of cargo parachute systems is especially difficult because of the low density/high altitude effect on airspeed of the aircraft. For example, an aircraft at an altitude of 25,000 ft., under standard atmospheric conditions, having an indicated airspeed of 130 knots is actually at a true airspeed of about 240 knots. Parachute opening forces increase with increased dynamic pressure, and dynamic pressure increases directly with the square of the aircraft true airspeed.
High dynamic pressures force a parachute to open very quickly, reducing opening time to fractions of a second. This situation can cause explosive, damaging and catastrophic openings. To prevent such events from happening during the deployment and opening phase of a parachute, methods to slow the opening have been devised. The methods are typically referred to as “reefing”. Reefing techniques usually involve restricting the initial diameter of the parachute canopy at it's skirt, where the suspension lines attach to the canopy. This is accomplished by skirt reefing or line reefing.
Skirt reefing employs a cord which is passed through rings attached to the parachute skirt at each suspension line towards the inside of the canopy. The cord length is pre-determined, depending on the degree of reefing required. A pyrotechnic device, with a time delay, is activated upon deployment from the aircraft. The device is used to provide a length of time in which the canopy is reefed, and then activate a blade to cut the reefing line at the skirt, allowing the parachute to fully open, or open to a second stage of reefing.
Line reefing forces the suspension lines to remain together in a bundle at a predetermined distance from the parachute skirt, thereby controlling the initial diameter of the parachute. Again, a time delayed pyrotechnic device is used to determine reefing time and to effect cutting of a retaining line to allow release of the suspension lines. A second method of line reefing makes use of the dynamic pressure exerted on a sail positioned at the middle of all the suspension line groups and attached to each suspension line group by a ring through which each line group passes. During the opening of the parachute, the pressure on the sail forces the suspension line groups closer together while the canopy begins to open and the entire canopy/payload system slows down. As the canopy inflates and spreads, the sail is forced to slide down the suspension line groups, allowing the canopy to continue to fully open.
With both types of reefing methods, when disreefing begins, inflation accelerates and high forces are exhibited upon the full open canopy with over inflation often occurring. Further, both types rely upon the proper functioning of a time-delayed pyrotechnic activated cutter device to cut reefing lines at pre-set times. Such devices add expense and weight to a parachute pack and necessarily include an explosive, a timer, and a sharp instrument.
There is a need for a reefing assembly for parachutes which reduces the shock load produced by rapid opening of the parachutes, and which does not require the use of a timed pyrotechnic device, nor a cutter device activated thereby for physically severing one or more reef lines.